The present invention relates generally to acoustic modeling, and more particularly, to a system and method for rendering an acoustic environment using more than two speakers.
Positional three-dimensional audio algorithms produce the illusion of sound emanating from a source at an arbitrary point in space by calculating the acoustic waveform which would actually impinge upon a listener's eardrums from the source. Systems have been developed to simulate a virtual sound source in an arbitrary perceptual location relative to a listener. These virtual acoustic displays apply separate left ear and right ear filters to a source signal in order to mimic the acoustic effects of the human head, torso, and pinnae on source signals arriving from a particular point in space. These filters are referred to as head related transfer functions (HRTFs). HRTFs are functions of position and frequency which are different for different individuals. When a sound signal is passed through a filter which implements the HRTF for a given position, the sound appears to the listener to have originated from that position.
Many applications comprise acoustic displays utilizing one or more HRTF filters in attempting to spatialize or create a realistic three-dimensional aural impression. Acoustic displays can spatialize a sound by modeling the attenuation and delay of acoustic signals received at each ear as a function of frequency, and apparent direction relative to head orientation. U.S. patent application Ser. Nos. 5,729,612 and 5,802,180, which are incorporated herein by reference, provide examples of implementation of a virtual audio display using HRTFs.
Stereo audio streams in which the left and right channels are developed independently for the left and right ears of a listener are referred to as binaural signals. Headphones are typically used to send binaural signals directly to a listener's left and right ears. The main reason for using headphones is that the sound signal from the speaker on one side of the listener's head generally does not travel around the listener's head to reach the ear on the opposite side. Therefore, the application of the signal by one headphone speaker to one of the listener's ears does not interfere with the signal being applied to the listcner's other ear by the other headphone speaker through an external path. Headphones are thus an effective way of transmitting a binaural signal to a listener, however, it is not always convenient to wear headphones or earphones.
Complications arise in systems which do not deliver the audio signal directly to the listener's ear. If a binaural signal is used to drive free standing speakers directly, then the listener will hear contributions from each speaker at each ear. The receipt of the signal intended for the right ear at the left ear and vice versa is referred to as “cross-talk”. It is necessary in such systems to compensate for or to cancel somehow the cross-talk so that the desired binaural signal is effectively applied to each of the listener's ears. The speaker cross-talk canceller does this by eliminating the positional cues related to speaker position and removing the interference of each speaker on the other.
A conventional implementation of a positional three-dimensional audio system includes a head-related transfer function (HRTF) processor followed by a speaker cross-talk cancellation algorithm. As previously described, the HRTF processor simulates the interaction of sound waves with the listener's head, ears, and body to reproduce the natural cues that would be heard from a real source in the same position. An impression that an acoustic signal originates from a particular relative direction can be created in a binaural display by applying an appropriate HRTF to the acoustic signal, generating one signal for presentation to the left ear and a second signal for presentation to the right car, each signal changed in a manner which results in the perceived signal that would have been received at each ear had the signal actually originated from the desired relative direction.